Renovator, was a comment removed? Because other than where you started, down, I don't see anybody ridiculing (or maybe you were talking about me? where I specifically said, IMO, and specifically addressed the question of whether or not they should have walked away?) Now, it has been side-tracked. But I don't think it had been until that point. Your answers on this forum are very helpful, but sometimes very objective. This person specifically asked both an objective and subjective question. They will get subjective answers. This person also has not stated, at all, where in the world they are located. And, whether mold might be a normal thing in their environment (to which I'd say, mold is everywhere, even deserts will have some sort of fungus growing there. It is the nature of nature). To say that those with asthma, etc could have problems even if it is non-toxic, while true, is misleading. Those folks will have problems no matter how their house is constructed in certain environments/ localities in the world. The OP said there was nothing found wrong with the ventilation and the suspected source was prior to the build and final construction. HOW MUCH MOLD (do you have a picture)? Finally, the usual way to remediate mold is actually to not use bleach (only). Mold remediation companies know what to use and would be better to hire than trying to DIY it and assume that you know that "bleach kills everything". I do second the suggestion to get a second opinion from another mold remediation company. (Just remember they will be interested in selling their services to you). However, I suspect at this point, in order to do that, you will need to re-enter contract with these sellers (to get access to their attic) and they may not trust you. Finally, you may want to get the RE opinions on this matter in the buying and selling forum since at least one of your questions pertains to future resale value.

Everybody should chill about mold. Is it damp in there? Smell moldy? Do the spots rub off? Have you tried to scrub the drawer with a scotch pad aND some mild detergent? Is your furniture pieces being stored in a damp basement? Outside in a shed?

Mold can only grow/survive when moisture is present and long term. The insulation can absorb moisture and help mold growth. If there is no moisture, there will be no mold. If there is mold, removing the insulation does not stop the mold, since the insulation only held the moisture. The remedy, if there is mold, is to remove the insulation, repair the moisture problem, create a well ventilated attic space and replace the insulation.

I see evidence of water/moisture/condensation on your framing lumber. Assuming that water isn't leaking in from above, I assume that warm moist air is moving up from the living space below, finding a cold surface and causing condensation. The damp materials then host mold. That would be my theory. If that was my house I would buy a case of cans of spray foam, the $50 gun to apply it, and go to town filling every nook and crany where warm air can move from below into the attic. If you start moving insulation around you may find wet or discolored insulation indicating where the big streams of air are coming through. If you have a door or hatch to the attic, those would be prime culprits but so would any opening in the sheetrock ceiling. I did this in my last two homes while they were being built which was a lot easier than after insulation was installed. Good luck.